1. Technical Field
The present invention relates to digital cameras, image processing apparatuses, and the like, and particularly to a digital camera, an image processing apparatus, and the like which track images of a target on input pictures.
2. Background Art
Recent digital cameras have an object-tracking function as positioning means for an auto-focus (AF), automatic exposure (AE), or backlight compensation function. Digital cameras are cameras for capturing still images and/or cameras for capturing moving images.
For example, a digital camera stores, as an amount of initial characteristics, color information on a specified image of a target. The digital camera then searches for a region having a similar amount of characteristics to the amount of initial characteristics on a picture captured after an image of the target is specified, to track images of the target. Specifically, the digital camera searches for a target region including an image of the target, by conducting template matching using color information or calculating the proportion of overlap between hue histograms.
A stationary camera for security purpose captures images in an almost fixed place and therefore can easily obtain information such as sunshine conditions and time changes. Thus, using the information such as sunshine conditions and time changes, the stationary camera for security purpose estimates a surrounding environment. According to the estimated environment, the stationary camera for security purpose is then able to change the color of template in template matching, for example.
However, in consumer appliances such as digital video cameras, which capture images of various targets in various environments, it is difficult to estimate a current environment. This means that in the case where there is a change in the surrounding environment (hereinafter referred to as “image-capture environment”) in which an image of a target to be tracked is captured, digital cameras are unable to follow the change in the environment and therefore cause a problem such as tracking an incorrect object or interrupting the tracking process. For example, in the case where a digital camera moves from a dark place to a bright place, tones for bright parts of a picture of the target to be tracked are lost (overexposure), which causes a problem that the template matching is not possible.
To address this problem, in the case where there is a change in the image-capture environment, conventional digital cameras corrects exposure, a color temperature, or the like on a captured picture, according to a changed environment (refer to PTL 1, for example). In the method disclosed by PTL 1, a luminance pattern of a picture is determined using a difference in luminance components between inside and outside of a detection region whose position on the picture changes. An exposure control is then performed using a photometry region set at a position suitable for the determined luminance pattern.
Furthermore, a method has been proposed in which a target is tracked using not hue information but a predicted coefficient calculated using each color of red, green, and blue (RGB) and a luminance Y, with the result that the target is tracked without fail even when there is a change in color attributing to illumination (refer to PTL 2, for example).